The proposed research is aimed at a definitive answer to a three-decade-old fundamental basic science question in bone tissue quality: "Is the cement line a weak interface?" Reliable predictions of fracture risk have been hampered by the significant overlap in bone density between normal individuals and those with osteoporotic fractures, suggesting the contribution of bone quality such as microstructure and biochemical composition. The cement line, existing in both cortical and trabecular bone tissue, is one of these important microstructures that have been suggested to play a crucial role in bone quality. Based on the morphology and composition of cement lines, it has been long hypothesized that cement lines serve as "weak interfaces" in bone tissue, suggesting an important role in fracture and microdamage processes of bone tissue. Therefore, I mechanical properties, especially the debonding strength of cement lines, are crucial in delineating the role of cement lines in the fracture toughness or bone fragility in bone tissue. However, there is almost no published literature on mechanical properties of cement lines. In this research, the following specific aims are proposed: Specific Aim 1: To manufacture and calibrate a new microtesting device for osteon pushout tests and perform osteon pushout tests using human femoral cortical bone specimens; Specific Aim 2: To perform microscopic compositional measurements using FTIRM and X-ray microanalysis SEM, and to measure the microscopic elastic modulus, using a nanoindenter, of the microtested osteons, cement lines and interstitial bone tissue; Specific Aim 3: To develop a constitutive law for cement lines and perform computational studies to extract the intrinsic interfacial properties (debonding strength and frictional coefficient) of cement lines. Based on the technology to be developed in this project, dependence of mechanical properties of the cement line on anatomic location, strain rate and aging can be determined. Thus, the outcomes from this research will provide new insights that will contribute to our general understanding of the etiology of osteoporotic fractures, and may lead to new perspectives and research in bone tissue quality.